In vibrating beam accelerometers, and in related sensors for measuring force or pressure, a force sensitive crystal controlled oscillator is used to convert a force into a corresponding shift of a carrier frequency. The oscillator includes a piezoelectric crystal having a size and shape adapted for a particular application. An electronic drive circuit maintains the piezoelectric crystal in oscillation by amplifying its output, and feeding the output back as an in-phase drive signal. The electronics for producing the drive signal may be either digital (square wave drive) or analog (sine wave drive).
The advantages of digital electronics are that they are relatively simple, they have low power consumption, and they self-limit for drive amplitude. However, digital drive signals are rich in harmonics that tend to excite overtones in the crystals, resulting in local nonlinearities termed "activity dips." Analog circuits are free of harmonics, but consume more power and require complex amplitude controls. For either digital or analog drive systems, performance in dynamic environments is sometimes compromised by the relatively low operating frequencies of the commonly used bending beam type crystals. The processing of frequency into an estimate of the force could be simplified if the crystals operated at much higher frequencies, e.g., 5-10 MHz, instead of the more typical values of 30-40 KHz.